The invention relates to an AC/DC or DC/AC converter system of the hexagon type. The Hexagon converter is a 12-pulse conversion circuit. If the 12 rectifying devices of such hexagon converter are diodes, the converter is suitable for AC/DC conversion only and it delivers a fixed DC voltage to its load. If the 12 rectifying devices are thyristors (or other controlable devices), the converter can perform on an AC/DC system, as well as on a DC/AC system. Moreover, it delivers, then, a controllable DC voltage. It is fully described in U.S. Pat. No. 4,255,784. The main attribute of the Hexagon circuit is that it can meet stringent specifications of the harmonic content of AC input currents drawn by the converter from the AC supply lines. Other 12-pulse converter circuits cannot meet most of these specifications without additional filtering circuits.
The harmonic performance of the Hexagon circuit hinges on the effective inductances which determine the commutation overlap angle .mu. of its semiconductor switches. The required commutation inductances can be designed into the Hexagon transformer in the form of leakage inductances, or can be provided externally in the form of "outboard" inductors. The preferred solution depends upon the rating of the converter as well as upon the specific size, weight and efficiency requirements.
An outboard inductor scheme which employs only two magnetic components to provide the equivalent of the required twelve commutation inductances is described in detail in U.S. Pat. No. 4,366,532.
However, the outboard inductor scheme there described in the cited patent is applicable without limitations only to converters which have controlled semiconductor switches. These switches (usually thyristors) start to conduct and initiate commutation of the preceding switch in the commutation sequence, when they are forward biased and when they receive the proper gate signal.
If the converter circuit is a diode rectifier, the outboard inductor scheme encounters limitations. Because diodes are not controllable semiconductor switches, the afore-mentioned outboard inductor scheme effects harmonic reduction only for commutation overlap angles .mu. not exceeding 30.degree.. In most instances, however, this is not sufficient because, in order to achieve the specified harmonic performance, the required overlap angle .mu. at maximum output current (resulting in the largest overlap angle) falls in the range of 30.degree. to 45.degree..
The problem arising with diode rectifiers in an Hexagon converter is solved according to the invention with an outboard inductor scheme which affects the commutation in such a way that an "illegitimate" commutation by an incoming diode cannot start at overlap angles less than the acceptable minimum.